Supplementary Information Concerning P&C Insurance Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Information For Property Casualty Insurance Underwriters [Abstract]
Deferred Policy Acquisition Costs	$ 135,249	$ 119,660	$ 91,983
Reserve for Losses and LAE	2,515,145	2,289,727	2,202,644
Unearned Premiums	987,681	887,344	820,676
Net Earned Premiums	1,186,420	1,100,345	984,087
Net Investment Income	89,293	79,451	68,718
Losses and LAE Expenses Incurred Related to Current Year	771,955	693,976	637,267
Losses and LAE Expenses Incurred Related to Prior Years	34,310	(28,528)	(64,669)
Amortization of Deferred Policy Acquisition Costs	184,731	165,045	129,977
Other Operating Expenses	233,230	234,096	223,516
Net Written Premiums	$ 1,271,330	$ 1,186,224	$ 1,043,860